Method for cast grain refinement of steel



March 14, 1967 TURNBULL ET AL 3,308,515

METHOD FOR CAST GRAIN REFINEMENT OF STEEL Filed Oct. 29, 1962 4 Sheets-Sheet 1 UJUFEE zwmmom N wE QFQMEEQ Fl TC ocbmtmwa .7.

[llllllllllllllll INVENTORS G. K. TURNBULL G. W, FORM f B F. WALLACE H xwzyigm; 4.1 m 8cm" ATTORNEYS March 14, 1967 TURNBULL ETAL 3,308,515

METHOD FOR CAST GRAIN REFINEMENT OF STEEL Filed Oct. 29, 1962 4 Sheets-Sheet 2 LTQUIDUS CURVE TEMPERATURE LEGENDZ CURVES o,b,c AND d REPRESENT POSSIBLE TEMPERATURE DISTRI- BUTIONS IN THE MELT AT I CRITICAL DEGREE OF UNDER' COOLING FOR INDEPENDENT NUCLE ATION.

FE G 5 DISTANCE FROM THE INTERFACE LIQUIDUS TEMPERATURE AND POSSIBLE TEMPERATURE DISTRIBUTIONS IN THE LIQUID AS A FUNCTION OF DISTANCE FROM THE INTERFACE DURING SOLIDIFICATION LIQUIDUS TEMPERATURE DISTRIBUTION CURVE O 0 F560 3 DISTANCE FROM THE INTERFACE G l 4 CONCENTRATION BUILD-UP AND LIQUIDUS DISTRIBUTION NEXT TO THE INTERFACE FOR SINGLE-PHASE INVENTORS G.K. TURNBULL ALLOY SOLIDIFICATION a w. FORM J. F. WALLACE ff KI mm; M WT a emf ATTORNEYS March 14, 1967 G. K. TURNBULL ETAL 3,308,515

METHOD FOR CAST GRAIN REFINEMENT OF STEEL Filed Oct. 29, 1962 4 Sheets-Sheet 5 MICROSTRUCTURE OF SMALL CONTROL CASTING SHOWING CONTINUOUS FERRITE NETWORK AND ORIENTATION EFFECT.

FIG. 7

GRAIN REFINEMENT OF AISI IOIB STEEL THROUGH ADDITION OF 0.25 Ti AND 0.30 ZrC.

INVENTORS GKTURNBULL G. W FORM J. E W/ILLHCE ATTORNEYS.

March 14, 1967 G. K. TURNBUL ETAL 3,308,515

METHOD FOR CAST GRAIN REFINEMENT OF STEEL Filed Oct. 29, 1962 4 Sheets-Sheet 4 FIG.8

SULFUR PRINTOF SMALL STEEL CASTING CON- TAINING 0.05 TITANIUM ADDITION.

FIG.9

SULFUR PRINT OF SMALL STEEL CASTING CON- 0 TAINING 0.5 /o VANADIUM BORON ADDITION. INVENTOR;

G.K. TURNBULL a, m FORM 0 J. E WALLACE ATTORNEY- United States Patent This invention relates to the treatment of molten low carbon steel whereby the as-cast structure of steel can be grain refined.

The object of this invention is the improvement of the cast structure of low carbon steel by the utilizationof additives to refine the grain structure to obtain a more heterogeneous product.

This may be accomplished by using the additives for the molten cast low carbon steel in the period between casting and solidification. These additives are selected on their capability to provide finely distributed centers for heterogeneous nucleation and/or on their potency for producing appreciable constitutional supercooling to interrupt continuous growth of the first formed crystallites.

Many of the important properties of metals are intimately connected with their microstructure, and control over the structure is, therefore, a means of arriving at a satisfactory performance of a given material. This invention is directed to the as-cast grain size of steel because of the marked effects that may be achieved in the physical and chemical properties. In castings, the size and shape of the primary grain structure is of particular importance, while in wrought material, the as-cast structure may be modified to a large degree by an appropriate combination of working and thermal treatment.

In the drawings:

FIGURE 1 is a schematic representation of the solidification sequence for a 1018 steel,

FIGURE 2 shows the effect of contact angle on the work of nucleation,

FIGURE 3 shows the concentration build-up next to the interface for a single-phase alloy solidification,

FIGURE 4 shows the liquidus distribution next to the interface for a single-phase alloy solidification,

FIGURE 5 shows the liquidus temperature and possible temperature distributions in the liquid as a function of distance from the interface during solidification,

FIGURE 6 shows the microstructure of small control casting showing continuous ferrite network and orientation effect,

FIGURE 7 shows the grain refinement of A181 1018 steel through addition of .25% titanium and 30% zirconium darbide,

FIGURE 8 shows a sulphur print of small steel casting containing .05% titanium addition, and

FIGURE 9 shows a sulphur print of small steel casting containing .5% vanadium boron addition.

Because of the complexity of low carbon steel solidification, it is necessary to define what is meant by ascast grain size. For this purpose the freezing sequence of a steel is schematically depicted in FIGURE 1. The first crystals rejected from a 1018 steel melt are of the deltairon type. When the peritectic temperature is reached, more than three-quarters of the liquid has solidified. The peritectic reaction will be initiated at the delta-iron boundaries and subsequent solidification of the remaining liquid will proceed by further growth of the already existing crystals rather than by independent nucleation in the melt. The bulk of the segregates retained in the liquid will finally be trapped in the last freezing areas surrounding the growing crystals, each of which originated from a separate delta-iron nucleus. It is the size of these crys- 3,368,515 Patented Mar. 14, 1967 tals, delineated by the segregates contained in the last solidifying liquid, which is referred to as the as-cast grain size. Although the grains defined in this manner are of the austenitic structure upon completion of the freezing process, they do not necessarily delineate the austenitic grain size, since the delta to gamma tnansformation may result in a subdivision of one as-cast grains into several austenitic grains.

The grain refinement of a cast structure may involve reduction in length of the columnar grains, a refinement of the width of the columnar grains, a reduction of the size of the equiaxed grains, or a combination of any of these processes. On a refiner scale, refinement of the structure may also involve a refinement of the dendrites. This latter aspect is of considerable significance in alloys, 01' in commercial metals in general, because of the fact that solute atoms not only tend to segregate to the grain boundaries, but also to the interdendritic spaces or last locations to solidify within a given dendritic grain. Heavy concentration of such segregates, particularly if present in the form of a continuous network, are not only detrimental to the mechanical properties in the as-cast condition per se, but also exert a marked effect on the type of phases and structures which form on cooling during the austenitic transformation. While the extent of grain boundary segregation is controlled largely by the as-cast grain size, microsegregation is determined primarily by the mode of dendritic solidification. The presence of segregates which diffuse only slowly are thus relatively immobile, permits the delineation of the phantom as-cast structure, and can therefore serve as a basis for identification of the latter. The position of high solute areas can either be determined directly by their physical and chemical properties, or indirectly by their effect on the solid state transformation.

A fine equiaxed grain structure during freezing can lead to improved mechanical properties, to a more uniform response in heat treatment, reduced anistropy and better founding properties compared to large columnar grains. Improvement in this direction has been attained with aluminum, magnesium, copper and anomalous eutectics, where the reduction in eutectic cell size, was the prime objective, while the proces of this invention refines the grain structure of low carbon steel by means of appropriate additions.

The achievement of a fine equiaxed, as-cast structure requires a nucleation to occur at a large number of uniformly distributed sites in the melt and the restriction of excessive growth of any one crystal. Difficulty encountered in achieving this dual objective originates from two sources; first, a positive temperature gradient normally exists in the liquid toward the center of the casting; and second, the temperature at which growth of crystallites can proceed is higher than the nucleation temperature. Consequently, since solidification in castings starts at the mold wall, the nuclei formed there in a pure metal tend to grow toward the center without interference, forming a columnar structure. This occurs, irrespective of the melt, because at any point ahead of the advancing solid liquid interface, the growth temperature is reached before the melt has cooled down to the nucleation temperature. While the availability of regions of easy nucleation is an essential feature of grain refinement, it is, in itself, insufiicient to promote a fine, equiaxed structure. Additional measures must be provided to make these potential nuclei become effective. This latter prerequisite is the grain growth aspect of grain refinement.

This invention is concerned with innoculation to promote rapid nucleation at many sites although this result can also be accomplished by chill action or by mechanical vibration. Inoculation is defined as the addition of can reduce the work of nucleation is best rationalized in terms of the interfacial energies acting on a fiat-sided particle, as shown in FIGURE 2. The theory of heterogeneous nucleation proposes that nucleation with a small degree'of undercooling which indicates a marked reduction in the work of nucleation, is favored in the case of a small contact angle. From the static equilibrium relation between the three interfacial energies 'YPL='YPs-i-'Ysr.' C 1' it follows, then, that nucleation can eifectively be facilitated with a particle that produces a large particle-liquid compared to the particle-solid interfacial free energy. The characteristics of the inoculant which governs this aspect and prime requisite thereof is a good matching between the crystal structure of the parent particle solid and the inoculating particle.

Grain growth restriction does not merely involve the V slowing down of the advancing solid-liquidinterface, but

a local enrichment of the liquid With solute atoms. This 7 is shown in FIGURE 3 for the case of single-phase alloy solidification. Since the liquidus temperature varies with composition, it must change near the advancing interface in a manner dictated by the concentration gradient shown in FIGURE 4. If the slope of the liquidus curve at the interface is steeper than thetemperature gradient in the melt, the liquid metal next to the interface is said to be constitutionally supercooled. The shaded area in FIG- URE 4 depicts this portion of the liquid. In order to achieve appreciable constitutional supercooling it is necessary that a solute addition be contained in the melt that produces a small distribution coeflicient K which is the ratio of solute in the solid, to solute in the liquid, and a large freezing range.

In FIGURE 5, a number of possible temperature distributions in the melt are superimposed on a given liquidus temperature curve. When a sufficiently shallow thermal gradient is obtained, as illustrated by curve of, independent nucleation in the melt, ahead of the solid-liquid interface can occur. Thus, the addition of an appropriate solute to the melt to achieve appreciable constitutional supercooling may serve as an effective means to fulfill one of the basic requirements of grain refinement, namely that new grains must form independently in the melt and thereby prevent the existing grains from further growth. If, by this means the growth of crystallities is interrupted early enough, the process of repeated independent nucleation will lead to the desired fine equiaxed grain. In view of the foregoing considerations, the process of this invention involves the addition to molten steel of p2 tential inoculants, and solutes possessing a strong ability for constitutional supercooling.

Additions selected generally fall into either one of two categories, an inoculant i.e. source foreign nuclei, or grain growth inhibitor while some additions were found to be potential grain refiners for both reasons. The choice of potentially suitable inoculants is made in the basis of good lattice registry with the base metal. The solidification of low carbon steel involves the selection of grain refiners with closecrystallographic matching with either d austenitic or delta iron. Since the delta phase constitutes the primary crystallites, attention is focused on the nucleation of this structure. it is assumed'that the development of many fine delta particles in the liquid was desirable in itself and would result in a finer austenitic grain size during the subsequent peritectic reaction. Among the crystallographically favorable substances, only those which were sufficiently stable in the parent liquid, are feasible. This excludes additions which melt at a temperature substantially lower than steel or which dissolve easily in molten steel.

Although most inoculants are soluble to some extent in liquid steel, the time interval of dissolution will permit an interval for effective inoculation. This requires a close control over the time elapsing between the introduction of the addition and actual solidification. While the dissolution aspects limit the effectiveness of an addition when large castings or high pouring temperatures are involved, it has the advantages that it provides for a clean surface which otherwise cannot be readily attained. The chemical reaction of a given inoculant with steel could be assessed to some extent from known phase relationships and known thermodynamic data. Some restriction on the selection of suitable inoculants is also imposed by difference of density between the addition and the steel which may lead to marked segregation by gravity, particularly when the particle size is coarse.

Grain growth inhibitors are selected on the basis of their potency to attain a large degree of constitutional supercooling per atomic percent of addition; Additives for the purpose of grain growth inhibition are not subject to many of the limitations imposed by an inoculating addition. The melting point of the solute becomes unimportant. The time interval elapsing between introduction of the addition and actual solidification is considerably less critical for grain growth inhibitors than for V The treatment necessary to produce the desired result sought in this invention comprises tapping molten steel from an arc furnace at a temperature of 2975 F. :25 F. deoxidizing each ladle or mold with 2 pounds of aluminum per ton of molten material, utilizing a large steel plunger to put the inoc-ulants and inhibitors in the molten steel and to maintain these additives below the surface of the melt until the reaction of the additives is completed; further additions of pig iron, ferro manganese, and ferro silicon may be made to compensate for oxidation losses, or sufiicient additional amount of aluminum to prevent oxidation of the additives and thereafter cooling the cast material to produce the grain structure desired. A mildly exothermic carbon-free hot-top material was added to the riser after pouring was complete. A typical time interval from the beginning of inoculation to the completion of the pouring was five minutes, including a pouring time of 45 seconds.

The castings were made from the same heat, with the first one without the addition of an inoculant for control purposes, while the subsequent castings were each inoculatlfzld with one of the substances listed in the following ta e.

Inoeulant Lattice Melting Added Crystal Parameter Density Point (Weight Structure (KX) gmjcmfi) (E) Percent) O.5TiO. NaCl i 4. 3183 i 4. 25 i 5, 880 0.5K 'l1F (See Note below) 0.52 3. 6090 6. 50 i 3, 380 3. 3065 4. 54 3, 128 4.4770 1. 55 I 1,486

NOTE.K TiF would dissociate at the high temperature to form Ti or TiC which would act as inoculants.

This Table III lists also the ingots to which sulphur was added intentionally to increase the sensitivity to sulphur printing.

TABLE II.THE CHEMICAL ANALYSES OF THE RESULTING LARGE STEEL CASTINGS Inoeulant Added 0, Mn, Si, S, P, A T Zr, Ca,

Percent Percent Percent Percent Percent Percent Percent Percent Percent;

Without Control- 225 80 69 030 018 TiC 257 1. 04 68 028 017 KaTiFen 281 99 G8 O21 015 Zr .243 97 69 .019 016 TL .300 .70 .61 .019 .015 C21 393 57 63 021 013 The titanium, zirconium and calcium additions were made because the metals are bodycentered-cubic and were intended to nucleate the delta phase. The carbides of these elements are either face-centered cubic or have a NaCl structure and could possibly provide heterogeneous nuclei for the austenitic phase. In addition to introducing each of the three inoculants as a pure metal, titanium was also added in the form of titanium carbide at K TiF salt. It was expected that K TiF would undergo an exothermic reaction on contacting the melt and thereby provide Ti or TiC through the reaction with the carbon in the steel in a finely distributed manner. The chemical analysis of the castings showed substantial variations in chemistry occurred because of the oxidation losses in the furnace and compensating additions to the furnace. The most critical of these was a range of carbon content from .225 to 393%.

In the treatment of small cast ingots, a series of untreated AISI 1018 melts were poured to ascertain the influence of a pouring temperature and cooling rate on the resultant as-oast structure. Despite a wide range of pouring temperature from 2800 to 3080 F., no discernible difference in the macrostructu-re resulted. The efiect of cooling rate after pouring was investigated, by pouring some samples into a green sand mold and pouring others into a refractory crucible which was placed into a furnace at 2200 F. and cooled to room temperature No apparent effect of the cooling rate on the macrostructure was observed and the conclusion was that unintentional slight variations in pouring temperature and cooling rate would not affect the results significantly.

Various quantities of up to one percent of a variety of substances were added to twenty pound ingots of 1018 steel. Table III lists these substances, together with their intended purposes and the degree of refinement achieved.

TABLE III Inoeulant Macro- Sulphur Added Grain Growth Inhib- (Weight structure (Weight Percent) itor (Weight Percent) Percent) (percent Equiaxed) Ch as FeCb. Altered. 25% Cb as E2011. 0. .40% Cb as FeCh 85. .80% Ch as FeCb..- Altered. .90% Cb as FeCb... Do. 1.00% Cb as FeCb D0.

.90% Ch as FeCb. .05% Ti as FeTi- .10% Ti as FeTi 50% Ti as Feli .60% Ti as FeTi 20% Ti as FeTi Ti as Feli .30% TiC. Altered 45% Ti as FeTi .45% TiC- Do. 30% Ti as FeTi 30% ZrC 80.

' Altered VB =Ferro-Vanadium-boron with 40 V and 6% B The assessment of the as-cast grain size was made through macroscopic inspection. If a columnar zone and an equiaxed zone could be distinguished from such an investigation, the structure was designated by a number indicating the relative length of the equiaxed region ex pressed in percent of the cross section. If a columnar zone was not revealed, the respective structure was referred to as altered. This investigation was followed up by macro-examination and sulphur printing in order to verify the macroscopic evaluation and permit closer examination of the grain size of the altered castings. This examination of the altered structures indicated that the castings with this appearance were grain refined in some instances but still large grained in others. The samples were etched with a selective reagent and subsequent microexamination of the control casting as shown in FIGURE 6 revealed that the delineation of the columnar mode of solidification apparent in the macrostructure was attributable to two features, namely a continuous ferrite network and an orientation effect. This FIGURE 6 shows a macrostructure in which the difierence in orientation of the Widmanstatten structure in adjacent columnar-grains is evident. It was not conclusively determined in all castings whether the ferritic network delineated the ascast or the austenitic grain structure. Titanium carbide and vanadium boron were particularly efiective in reducing the columnar length, while tungsten, AlCo and AlNi had no effect on the columnar structure. A grain growth inhibiting eifect was achieved with small solute additions of columbium and titanium. Although solute additions of boron, zirconium and vanadium, exerted a very pronounced effect on the resulting macrostructure appearance, a positive identification of the as-cast grain size by microscopic examination was not possible in these castings.

A typical example of the combined action of an inoculant and a grain growth inhibiting agent is shown in FIG- URE 7 in which the casting shown contained additions of .25 titanium and 30% of zirconium carbide to produce a pronounced reduction of the columnar structure.

The utilization of the sulphur printing technique provides additional information and further support for the previously assessed as-cast grain size. Although this method delineates the mode of dendritic solidification rather than the grain boundaries of the different cast grains, it is possible to observe the change from columnar to equiaxed grains in many instances. An example of this is shown in FIGURE 8 illustrating a sulphur print of a casting containing .05% titanium. The macrostructure of this ingot is approximately 65 percent equiaxed. Throughout the columnar zone the dendrites are orientated with their major axes parallel to the direction of heat fiow. In the equiaxed center zone, on the other hand, their orientation is random.

Sulphur printing proved particularly successful with castings containing sulphur additions when employed with those structures in which the as-cast grain size could not be identified by a macroscopic etching technique. The results were less conclusive when deliberate sulphur additions were not made and the sulphur content in the castings was low. FIGURE 9 shows a casting to which s,-sos,5 15

.5% VB was added for the purpose of inoculation. On thebasis of the initial macroscopic examination, this casting was classified as having an altered" structure. The sulphur print, however, brings out a transition from columnar to equiaxed solidification and permits classifica tion of this casting as 75 percent equiaxed, thereby indicating appreciable grain refinement. It appears that an altered macrostructure is generally indicative of a 'marked refinement of the as-cast grain size.

The foregoing description of this process of treatment indicates some of the major difficulties encountered when grain refining the as-cast structure of steel. The identification of the grain size prevalent at the end of freezing requires a detailed and thorough analysis of all those facets which are inherited from the as-cast grain boundary or dendritic structure, and which persist during the allotropic transformation occurring on cooling to ambient temperature. The relatively high melting point of steel restricts the number of potentially suit-able inoculants because of the stability requirement imposed on such substances. In addition, some of the very promising additions which could act as potent grain growth inhibitors oxidize readily at the high temperatures involved and are either rejected into the slag or remain as oxides in the melt. In either case, the potential of these additions to produce appreciable constitutional super cooling is markedly reduced. The means utilized by this process for introducing the addition to the molten steel provides a fine uniform distribution and avoids too much reactivity of the grain refining additives with the prevailing atmosphere and the constituents in the steel. Additives that appear promising on the basis of theoretical considerations will only produce successful refinement of the as-cast structure, if properly added to and alloyed to remain in the melt in'an active state.

Of the inoculants added to a variety of 20 pound castings of A181 1018 steel, titanium carbide and vanadium .boron proved to be the most effective in producing a fine equiaxed center zone of appreciable length. A considerable grain growth inhibiting effect was achieved with small amounts of columbium or titanium. Simultaneous additions of titanium and titanium carbide resulted in the most consistent grain refinement of these small castings. A series of castings inoculated with various amounts of these two additions were therefore subjected to room temperature tensile tests which showed an improvement in strength and a loss in ductility when compared to the standard casting. The embrittlement was attributed to grain boundary precipitation. Having more particularly described as-cast grain growth and the conditions found necessary for utilizing additives to obtain the most advantageous results, what is claimed is: 1. The method of treating molten low carbon steel comprising, casting the molten material at a temperature of approximately 2975 F., deoxidizing the cast material with aluminum, plunging and maintaining below the surface of the molten material, approximately 20% by weight of titanium as a grain growth inhibitor for restricting growth of crystals present in cast material and approximately 20% by weight of titanium as a grain growth inhibitor or restricting growth of crystals present in cast material and approximately 20% by weight of titanium carbide as an inoculant to provide additional nuclei for new crystal growth, adding an additional .amount of aluminum to prevent oxidation of the additives, and gradually cooling the molten material whereby independent nucleation is promoted and normal crystalline growth of the cast material isinterrupted and reduced to improve the as-cast grain structure of the steel wherein the macrostructure of the cast steel is at least 80% equiaxed. 2. The method of treating molten low carbon steel comprising, deoxidizing the molten material, plunging below the surface of the molten material a comminuted metal additive of approximately .45% by weight of titanium carbide as .311 inoculant to provide additional nuclei for new crystal growth and also acting as a grain inhibitor to restrict the growth of crystals present in the cast material, adding an additional amount of deoxidizer V of approximately 2975 F., deoxidizing the cast material with aluminum, plunging and maintaining below the surface of the molten material approximately 30% by weight of titanium as ferro-titanium as a grain growth inhibitor for restricting growth of crystals present in cast material and approximately 30% by weight of zirconium carbide as an inoculant to provide additional nuclei for new crystal growth, adding an additional amount of aluminum to prevent oxidation of the additives and gradually cooling the molten material whereby independent nucleation is promoted and normal crystalline growth of the cast material is interrupted and reduced to improve the as-cast grain structure of the steel wherein the macrostructure of the cast steel is at least equiaxed.

4. The method of treating molten austenitic steels comprising, deoxidizing the molten material, plunging below the surface of the molten material a comminuted additive of ferro-columbium containing approximately .40% by weight of columbium as a grain inhibitor for restricting growth of crystals present in the molten material and also acting as an inoculant to provide additional nuclei for new crystal growth, adding an additional amount of the deoxidizer in suflicient quantity only to prevent oxidation of the additive and gradually cooling the molten material whereby normal crystalline growth in the molten material is interrupted and reduced therein to improve the grain structure of the steel and the macrostructure of the treated steel is at least equiaxed.

5. The method of treating molten low carbon steel comprising, deoxidizing the molten material, plunging below the surface of the molten material a comminuted metal additive of approximately 50% by weight of ferrovana-dium-boron containing 40% vanadium, 6% boron as an inoculant to provide additional nuclei for new crystal growth and also acting as a grain inhibitor to restrict the growth of crystals present in the cast material, adding an additional amount of deoxidizer in sufiicient quantity only to prevent oxidation of the additive and gradually cooling the molten material whereby independent nucleation is promoted and normal crystalline growth in the molten material is interrupted and reduced to improve the as-cast grain structure of the steel wherein the macrostructure of the treated steel is at least 75% equiaxed. fine-alarm i References Cited by the Examiner UNITED STATES PATENTS 1,596,888 8/1926 Pacz 22215 1,814,584 7/1931 Bost et al. 22-215 2,221,783 11/1940 Critchett 7558 2,291,842 8/1942 Strauss 75--58 2,479,097 8/1949 Buchanan 75--58 2,778,079 1/ 1957 Carney et al. 22215 2,809,109 10/1957 Field 22215 2,840,872 7/1958 Bidner et al. 22215 3,019,497 2/1962 Horton et al. 22215 J. SPENCER OVERHOLSER, Primary Examiner.

MARCUS U. LYONS, MICHAEL V. BRINDISI,

Examiners.

E. MAR, Assistant Examiner. 

2. THE METHOD OF TREATING MOLTEN LOW CARBON STEEL COMPRISING, DEOXIDIZING THE MOLTEN MATERIAL, PLUNGING BELOW THE SURFACE OF THE MOLTEN MATERIAL A COMMINUTED METAL ADDITIVE OF APPROXIMATELY .45% BY WEIGHT OF TITANIUM CARBIDE AS AN INOCULANT TO PROVIDE ADDITIONAL NUCLEI FOR NEW CRYSTALS GROWTH AND ALSO ACTING AS A GRAIN INHIBITOR TO RESTRICT THE GROWTH OF CRYSTALS PRESENT IN THE CAST MATERIAL, ADDING AN ADDITIONAL AMOUNT OF DEOXIDIZER IN SUFFICIENT QUANTITY ONLY TO PREVENT OXIDATION OF THE ADDITIVE AND GRADUALLY COOLING THE MOLTEN MATERIAL WHEREBY INDEPENDENT NUCLEATION IS PROMOTED AND NORMAL CRYSTALLINE GROWTH IN THE MOLTEN MATERIAL IS INTERRUPTED AND REDUCED TO IMPROVE THE AS-CAST GRAIN STRUCTURE OF THE STEEL WHEREIN THE MACROSTRUCTURE OF THE HEATED STEEL IS AT LEAST 70% EQUIAXED. 